Metal clad laminates for circuit board construction are constructed by laminating a polyimide film to a metal foil with an adhesive layer in between. The adhesive layer may consist of conventional adhesives (acrylates, epoxides, polyamides, phenolic resins, etc.) where the adhesive is cured during the metal foil lamination. However, these conventional adhesives do not usually possess the high temperature heat stability of the base polyimide dielectric, and the strength of the adhesive bonds in the multiplayer laminate structure deteriorates rapidly when subjected to elevated temperatures. These adhesives also show high electric loss in high speed circuit layers due to the high loss tangent of these adhesives.
As electronic packaging becomes more sophisticated and the desire for thinner, smaller, light weight, flexible, electronic components with low CTE, low moisture absorption, high temperature heat stability and high modulus have necessitated the elimination of the adhesive layer.
One means to an adhesiveless laminate is to coat a high modulus polyimide film core layer with a thin layer of polyamic acid precursor solution on both sides, dry this layer, and finally imidize the applied coating to create a thermoplastic polyimide. Copper foil is then laminated with heat and pressure to create a double sided copper clad laminate.
A polyimide precursor can also be coated directly onto copper foil and then cured to create a polyimide film with copper foil on one side. This is a common method of making adhesiveless copper clad films but can create polyimide clads with copper on just one side. Another method of making adhesiveless polyimide copper clad laminates is to start with a standard rigid polyimide base film. A thin metal layer is deposited, typically by sputtering or vapor deposition to improve adhesion between the metal and the polyimide. Then the copper is electroplated up to the required thickness to create a double sided copper clad laminate.
Each of these suffers from one or more of the following disadvantages: requisite additional steps as opposed to lamination of a metal foil(s) to polyimide film, inferior adhesion of the metal foil to the polyimide when an adhesive is used, multiple layers of polyimide add to overall the thickness of the construction.
For the forgoing reasons, a need exists for polyimide films that can be directly laminated/adhered to metal foils without using an adhesive to providing thinner, flexible electronic components with low CTE, low moisture absorption, high temperature heat stability while maintaining good mechanical and electrical properties.